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6 - Reflection & Transmission

6 - Reflection & Transmission. Spectral Reflectance Curves. My hand illuminated by different light sources. Low Pressure sodium - two monochromatic lines near 590 nm - very “orange” High pressure sodium - broader SPD - “pinkish”

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6 - Reflection & Transmission

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  1. 6 - Reflection & Transmission

  2. Spectral Reflectance Curves

  3. My hand illuminated by different light sources Low Pressure sodium - two monochromatic lines near 590 nm - very “orange” High pressure sodium - broader SPD - “pinkish” Incandescent - very broad SPD, but tends to look reddish compared to sunlight Coated Incandescent - coating makes it appear more like sunlight

  4. Color-coding & “False Color” • Normally represent green with green, etc. • Sometimes color shifts occur if not faithfully reproduced - green versus greenish • Sometimes gross errors occur in coding - green original versus non-green representation • Can also deliberately re-code: especially useful in representing light the human eye cannot see

  5. Landsat 7 scene was imaged in 4 different wavelengths: Band 1 Blue 470 nm Band 2 Green 555 nm Band 3 Red 660 nm Band 4 IR 880 nm

  6. Transmission Through Filters (more) Most colored filters - dyes (organic & inorganic) “Color-blind” filters: Black or gray absorbers - photo B&W negatives Scatterers - ground glass

  7. Filters in Series T1 T2 T3 T4 Two filters with transparencies T1 and T2 combine so that the net transmission T12 is just: T12 = T1T2 If we had 3 filters, the result would be T123 = T1T2T3 If we had N filters, we would getT123…N = T1T2T3…TN If all the filters in our series were identical, this simplifies to: TN = TN I0 I1 I2 I3 I4 I1=I0T1 I2=I1T2=I0T1T2 I3=I2T3=I0T1T2T3 I4=I3T4=I0T1T2T3T4 So T4=I0T1234 where T1234=T1T2T3T4

  8. Example, 8 identical filters, each with T= 1/2. The net transmission would be: • T = (1/2)8 = (1/28)  0.0039  0.4% • I0 I • I = I0e-ax = I0e- • a = absorption coefficient per unit thickness the material • x = thickness • = ax the “optical depth” e = 2.7182….. Effect of increasing thickness -(Bouguer’s Law) Effect of increasing absorption coefficient (Beer’s Law)

  9. Smooth and Rough Surfaces

  10. Driving on a wet road at night………. Hard to see your own headlights on the road - can’t see painted lines! Glare from headlights of oncoming cars reflecting off the water on the road Very bad if angle of incidence > Brewster’s

  11. Color Effects The net transmission through a series of filters must be calculated at every wavelength: T12,440nm = T1,440nmT2,440nm T12,550nm = T1,55nmT2,550nm etc. or in general, T = T1,T2, Similarly, the reflection at a surface must be done in the same way

  12. Older color cameras used dichroic filters

  13. Modern digital cameras use charge-coupled devices CCDs These are intrinsically B&W detectors, but can be “coded” to make color images

  14. The Future: Digital devices that “tag” the photon with its energy, hence wavelength - “3D” detectors!

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